From PinataIsland.info, the Viva Piñata wiki

In a multiplayer game, the other players(non owner(s) of the garden) can gain magic and fill their circle with color by planting seeds, watering plants, or fertilizing plants. Once the circle is full of color (magic), you can then use the magic in multiple ways, such as healing sick pinatas, making a pinata happy, filling a pinata's Candiosity meter
, and tinkering items at 100% efficiency.